I had the same trouble figuring out which is more worth it for me:
16gb RAM?
Fusion drive?
27 inch screen with a better GPU (660M vs 650M)?
Of course I must leave out those "best" options like 27 inch + 675M or even 680MX and stuff, as it'll completely kill my budget...
Here's my analysis:
1. 27 inch + 660M vs 21.5 inch + 650M: I've been to an Apple reseller checking out and comparing the old 21.5 and 27 inch iMacs there. The biggest take away for me besides that the magic mouse absolutely sucks (haha), is that the 27 inch while is really, really nicely HUGE, it's impractical for myself. My eyesight is not exactly good and I don't use glasses, also I prefer to read bigger fonts. These reasons make me prefer to have the screen slightly closer to me. And this in turn makes the 27 inch too big for me, as I need to constantly move my eyeballs or even my neck to see everything. If I move the screen too far away everything will be a bit hard for me to read.
Another important issue is the GPU on gaming. The top specs of 27 inch is out of question so I can only compare 650M vs 660M. From various benchmarking I know that running 1920*1080 on 650M is much more reliable and smoother than running 2560*1440 on 660M. And I seriously don't want to like running games on non-native resolutions.
Conclusion: 21.5 inch for me.
2. 16gb vs 8gb RAM: I don't run photoshop and other RAM demanding apps, I mostly read PDF and make Keynotes and use virtual machines, I also hate leaving unopened background apps and Safari tabs open (makes me feeling lost), so I *think* 8gb will be enough for me. Though I must admit 4gb is hardly enough for my 2012 MBA, so I do have a slight fear that 8gb will not be enough for the iMac. Then again, I expect that even if my current RAM usage doubles up, 8gb will still be (just enough) for me.
Conclusion: May not upgrade to 16gb; really depends on my mood when I put my order
3. Fusion drive vs just HDD: now this is a very good question. Having used 7200 RPM, 5400 RPM and SSD before (well, I think most of us do so
) I can tell you how the phrase "never look back" really applies on the SSD. Booting speed is one thing, but being able to instantly search through your files and PDF/XLS contents can speed up your work efficiency massively. However I'm not very sure if my Fusion drive has over 600gb of files will it still be able to search file content that fast...
Conclusion: I'll probably upgrade to Fusion drive...as once you get used to SSD like myself got used to the MBA, you'll hardly tolerate all those "LOADING..." or "SEARCHING..." screens anymore.